The unusual combination of anisotropic and fluid behavior of liquid crystals has resulted in their use in electro-optical switching and display devices, where their electrical, magnetic, elastic and/or thermal properties can be utilized to cause changes in alignment. Optical effects can be achieved, for example, with the aid of biorefringence, the inclusion of dichroically absorbing dyes ("guest-host mode") or light scattering.
In order to satisfy the constantly increasing practical requirements in the various areas of application, there is a constant demand for new improved liquid-crystal mixtures and thus also for a large number of mesogenic compounds of various structures. This applies both to applications in which nematic liquid-crystal phases are used, and to those having smectic liquid-crystal phases.
The processes for the preparation of the components of such liquid-crystal mixtures also have to meet constantly increasing requirements, in particular with respect to the effects on ecology, but also with respect to process economy. The liquid-crystal mixtures virtually always comprise at least two different classes of substance, and very frequently at least two homologs differing, for example, in the chain length of an alkyl or alkoxy substituent are very frequently found in a certain class of substances (see, for example, EP-A 497 176, U.S. Pat. No. 5,026,506, EP-A 495 686, EP-A 319 167, EP-B 317 587, EP-A 316 181, EP-A-315 958).
Attempts have therefore already been made to find ways in which the syntheses of various classes of substance, but ones which contain common part-structures, can be accomplished on the basis of the same precursors.
EP-A 354 434 describes derivatives of boric acid, including boronic acids, which are reacted with certain halogen compounds with catalysis with metal compounds to give liquid-crystal compounds.
However, a significant disadvantage of the compounds described in EP-A 354 434, which impairs inexpensive production and is ecologically unacceptable due to additional processing steps, is the fact that on further conversion into liquid-crystal compounds for the preparation of more than one homolog of a substance class, in each case more than one boric acid derivative or halogen compound is necessary, since the substitution pattern of the desired target molecules is already defined in the starting compounds for the boric acid derivatives or hydrogen compounds, and homologization is no longer possible in the subsequent reaction steps.
For the preparation of different classes of substance--which is taken to mean here, for example, not only phenylpyridine versus phenylpyrimidine, but also, for example, 5-alkyl- versus 5-alkoxy- versus 5-alkoxycarbonyl- versus 5-alkylcarbonyloxypyrimidine, since significant differences with respect to mesogenic properties, stability, synthesis and thus not least also of the precursors exist--different precursors must in each case be prepared in accordance with the prior art.